I need some guidance what size drill bit? 8mm? also, on the topic of leaks my filling adaptor worked lose the other day, I'm not a fan of the t bar design, so was wondering about replacing it for a snap fit connector? similar to those used on daystates?

My air arms s400 has developed a leak from the pressure gauge. It stops when the gun goes down to 150bar, so i suspect one of the seals has failed or it's lose. My gun is AT so is it just a case of drilling out the AT? then I presume there is a hex bolt?